Antilock braking systems have become increasingly popular in both passenger cars and heavy duty vehicles. These systems require accurate wheel speed signals which vary in accordance with vehicle wheel speed. Such wheel speed signals are generated by wheel speed sensors. These wheel speed sensors include a serrated tone wheel, which is mounted for rotation with the vehicle wheel; a sensing head, which is mounted to a non-rotating part of the vehicle adjacent to the tone wheel; and a conditioning circuit, which receives the signals generated by the sensing head. The sensing head consists of a coil wound around a cylindrical permanent magnet, the end face of which is in close proximity with the tone wheel. The tone wheel is made out of a ferromagnetic material and is provided with serrations circumferentially spaced equally around the tone wheel. As the tone wheel turns in front of the sensing head, the flux density inside of the coil will change depending on whether the end face of the permanent magnet is facing a tooth or a serration defined by two adjacent teeth. This flux change induces an alternating voltage in the coil, the frequency of which is equal to the rate of change of the teeth and gaps of the Tone wheel rotating past the end face of the magnet. Accordingly, the frequency of the output of the sensor is a direct measure of the vehicle wheel speed.
Since wheel speed sensors are critical components of the adaptive braking system, it is necessary to test the integrity of the speed sensors at regular intervals, at and after engine start up. The sensing head is normally tested for an open circuit through the coil or a short circuit across the sensing head, although sometimes speed sensors are checked only for an open circuit, the most common fault. The most common way of Testing speed sensing heads is by injecting a test signal through the head. The test is controlled by a microprocessor which receives the wheel speed signal through a digital input port during normal operation of the sensor. The test signal is transmitted by the microprocessor to the sensing head, and a return test signal is received at an analog port of the microprocessor. This return signal indicates whether the sensing head is in satisfactory operating condition or is faulty. The main disadvantage of this prior art testing arrangement is that it ties up microprocessor resources; that is, both a digital and an analog port are required for each speed sensor, with the analog port used only during the test sequence.